The present invention relates to a portable radio communication device having circularly polarized antennas effective for satellite communication.
In mobile communication through portable radio communication devices (portable telephones) and the like, linearly polarized waves belonging to frequency bands of 800 MHz, 1.5 GHz, and 1.9 GHz are currently used. Portable telephones using medium-earth-orbit and low-earth-orbit satellites have been proposed by communications system companies in recent years, and for such mobile satellite communication, the following frequency band allocation systems has been schemed. One system allocates a frequency band of 1.6 GHz for uplink communication from portable telephones on the ground to satellites and a frequency band of 2.4 GHz for downlink communication from satellites to portable telephones on the ground. Another system allocates a frequency band of 1.6 GHz for both uplink and downlink communications. For such communication, circular polarization is generally used to guarantee the quality of radio communication lines.
Proposed as an exemplary antenna construction is, as shown in FIG. 7, a folding antenna array 25 (see xe2x80x9cNon-geostationary geostationary Satellite Communication Systems of the Worldxe2x80x9d, ITU Research, No. 261/262, New Japan ITU Society, August 1993, p. 36). The folding antenna array 25 uses a transmitting microstrip flat antenna (hereinafter referred to as the xe2x80x9ctransmitting flat antenna) 22 and a receiving microstrip flat antenna (hereinafter referred to as the xe2x80x9creceiving flat antenna) 23 for satellite communication.
The folding antenna array 25 will be described. For the sake of explanation, a construction proposed by the ODYSSEY system (TRW Corp. of the U.S) that is disclosed in the aforementioned literature xe2x80x9cITU Researchxe2x80x9d will be taken as an example. A portable telephone 92 shown in FIG. 7 has the folding antenna array 25 that has the transmitting flat antenna 22 and the receiving flat antenna 23 arranged on a dielectric. The folding antenna array 25 can be arbitrarily folded with respect to the portable telephone 92 at a folding angle xcex1 ranging from 0 to 180xc2x0 through hinges 85.
In this case, the transmitting flat antenna 22 and the receiving flat antenna 23 communicate with a satellite using frequencies f1, f2, respectively. In the case of a satellite communication system using the same frequency f1 for both transmission and reception, only the transmitting flat antenna 22 can be used for both transmission and reception. Generally, the frequency f1 uses a frequency band of 1.6 GHz and the frequency f2 uses a frequency band of 2.4 GHz.
However, a small-sized flat antenna using a dielectric sheet has a narrow bandwidth, and the resonance frequency of the flat antenna varies due to interference between itself and surrounding dielectrics such as a housing and a human body. As a results the transmission efficiency and reception sensitivity of the antenna have been impaired. Especially, in the case of communicating with a low-earth-orbit satellite, loss in antenna gain caused by the head of a human body is addressed as a problem.
Further, communication sensitivity at low elevation angles is lost in the flat antenna when the main beam is directed toward the zenith (at an elevation angle of 90xc2x0). While the use of either the flat antenna or the helical antenna has been proposed in mobile communication through a communications satellite, satisfactory communication sensitivity can be obtained only either when the satellite is at low elevation angles or when the satellite is at high elevation angles due to restrictions on the directivity specific to each type of antenna.
The present invention has been made to overcome the aforementioned problem by preventing the nearing of an antenna element toward a human body (especially, the head) when the user uses a portable radio communication device. That is, in a circularly polarized antenna that is mounted on a portable radio communication device, a communicating section is arranged between the portable radio communication device and the antenna element.
Further, the present invention is characterized as providing two circularly polarized antennas on a portable radio communication device, and the main beam radiating direction of one of these antennas is made different from that of the other.